Cobalt-based alloys are used in many wear or abrasion-intensive applications because of their excellent wear resistance and ability to alloy well with many desirable alloying elements. One potential problem with Co-based alloys is their corrosion resistance when exposed to a corrosive medium, such as seawater, brackish water, mineral oil-based hydraulic fluids, acids, and caustics. One way that Co-based alloys have been designed to display improved corrosion resistance is by including Mo and Cr. But the simultaneous presence of C in many Co-based alloys can reduce the efficacy of these alloying elements by forming carbides. Therefore, the C concentration in Co-based alloys traditionally has been decreased to allow the Mo and Cr additions to impart improved corrosion resistance to the alloy. The lowered C concentration, however, has the undesirable effect of lowering the alloy's overall hardness, thereby reducing the alloy's wear resistance. So Co-based alloys for use in wear environments usually have a C content over 0.1%.
Further, Co-based alloys are particularly useful in high temperature applications because of Co's high melting point. But forming entire components using Co-based alloys is cost prohibitive. For example, it is cost prohibitive to form a 500 lb. component from a Co-based alloy, whereas forming a Co-based overlay on a Fe-based substrate is much cheaper. Therefore, to still take advantage of Co-based alloys' desirable properties, one common use of Co-based alloys is as a surface treatment, e.g., a coating or overlay, on substrates. Because of the high heat involved in applying Co-based alloys as a surface treatment, preheating the substrate is often required to avoid cracking of the overlay as it cools. Preheating is difficult or commercially impractical when the Co-based alloy is being applied to large substrates. Furthermore, substrates made of heat treated metals may not be heat-treatable at all because such a procedure would cause distortion or degradation of intended substrate properties. Therefore, to successfully treat a substrate surface with a Co-based alloy without preheating, the alloy must have sufficient flow characteristics in molten form and ductility during and after solidification. It must also have thermal characteristics compatible with deposition onto a relatively cooler substrate without preheating.
U.S. Pat. No. 5,002,731 discloses Co—Cr—Mo—W alloys with C and N for improved corrosion and wear resistance. These alloys have a low C content, so they lack resistance to abrasive wear due to insufficient precipitated carbide particles.
U.S. Pat. No. 6,479,014 discloses higher C alloys of Co—Cr—Mo and Co—Cr—Mo—W for saw teeth. These are designed for both wear and corrosion resistance, but they can be brittle from excessive precipitated carbides, and significantly, from the formation of intermetallic phases.